Usually, in the ground, many metal pipes, such as a water pipe, a gas pipe, and a drain pipe, or cables, such as a telecommunication cable and an electric power cable, are buried (hereafter, except for the conventional example portion, these are named generically and it is only described as buried metal).
A buried metal is not simply buried under the horizontal direction or the perpendicular direction linearly.
A buried metal is crooked horizontally or vertically, or branches, is in the state which crossed intricately mutually and carried out congestion, and is buried.
Thus, since much buried metal is in the ground, for the accident prevention by construction of other companies and for the efficient control of maintenance of the buried metal of one's company, by un-excavating, the technology for detecting the position (a horizontal position and the burial depth) of in the ground buried metal is proposed.
As typical prior art for detecting the position of a buried metal, there are a radar which searches underground, and an electromagnetic guidance pipeline detector called a pipe locator.
The detection principle of a radar which searches in the ground detects in the ground metal pipe by emitting an electric wave in the ground, receiving the reflective wave from the metal pipe currently buried in the ground, and performing signal processing and conversion to signals.
Since an electric wave is reflected from the face where physical properties change, in the ground structures, such as a metal pipe, a nonmetallic pipe, and a cave, can also be detected by this method.
On the other hand, as shown in FIG. 6 (a)-(c), the detection principle of the electromagnetic guidance pipeline detector (brand name: pipe locator) of nonpatent document 1 is as follows.
If an alternating current is sent through metal pipe 51 which is buried in the ground 50, a concentric circle-like magnetic field (it is considered as magnetic field H) will occur centering on this metal pipe 51 (refer to FIG. 6 (b)).
Magnetic sensor 52a of the receiver which is on the ground detects this magnetic field H, that magnetic field H is calculated, and induction voltage is calculated further.
The position of metal pipe 51 can be detected from the position where this induction voltage serves as the maximum (FIG. 6 (c)).
The burial depth of metal pipe 51 is calculated from this induction voltage.
Therefore, the object detected is limited to the metal pipe which has conductivity.
Thus, the pipe locator is constituted by the receiver provided with the transmitter for sending current through a metal pipe, and the magnetic sensor which detects magnetic field H generated when current flowed.
Furthermore, the method of sending current through a metal pipe from a transmitter has two kinds, direct method and a induction method.
As shown in FIG. 7, in direct method, the transmitter 53 is connected to the portion of metal pipe 51 exposed on the ground, or transmitter 53 is connected to the metal pipe 51 currently laid in the ground 50 via lead 54, and current is sent.
In addition, the leak current which flows into the earth 50 from the metal pipe 51 is constituted so that it may feedback to a transmitter 53 through a earth 55.
On the other hand, as shown in FIG. 8, in induction method, emitting an electric wave towards in the ground 50 from transmitter 53a which installed on the ground, generating a magnetic field and the current by electromagnetic induction is sent through metal pipe 51 of in the ground 50 by non-contact.
The magnetic field by the generated induced current receives with receiver 53 provided with magnetic sensor 53a which is on the ground, and is detecting a position, burial depth, etc. of the metal pipe currently buried from the amplitude of the magnetic field component.
In direct method as shown in FIG. 7, since current can be sent only through the metal pipe for detection, the value of magnetic field H to generate also becomes large, and detection accuracy is good compared with an induction method.
However, in direct method, since a transmitter must be directly connected to a metal pipe, it is hard to apply to the metal pipe which does not have an exposed portion on the ground.
On the other hand, as shown in FIG. 9, an alternating current is sent through line buried metal body 61 as a method of investigating the burial position and depth of line buried metal body 61, detection coil 62 detects change of magnetic field H which this generated, at least two burial detection measuring methods were adopted, and it applied for the invention which amended detection measured value based on the measurement result by these previously (patent documents 1).
Hereafter, this is explained.
As shown in FIG. 9, line buried metal body 61 sufficiently long in the shape of a straight line is laid under the position of perpendicular distance y from detection coil 62, and it is assumed to this line buried metal body 61 that the current of I sin (ωt) is flowing.
Then, when detection coil 62 parallel to surface of the earth has been arranged near right above this line buried metal body 61, generally electromotive force Eh induced by detection coil 62 is expressed with a following formula (1).Eh=k{(y)/(x2+y2)}Iω sin(ωt)  (1)
Here, x is the horizontal distance from right above line buried metal body 61 to detection coil 62, and k is a fixed number which becomes settled with detection coil 62.
From the above-mentioned formula (1), electromotive force Eh becomes the maximum in a right above line buried metal body 61 position, and the value is proportional to the current which flows through line buried metal body 61, and is in inverse proportion to perpendicular depth y.
Then, as a method of measuring burial depth y of line buried metal body 61, when detection coil 62 is horizontally moved from a right above line buried metal body 61 position (x=0), electromotive force E0 induced by detection coil 62 is expressed with a following formula (2) that what is necessary is just to substitute x=0 for the above-mentioned formula (1).E0=k{(1/y)}Iω sin(ωt)  (2)
Subsequently, after moving only distance x horizontally (x direction) from the right above position (x=0) of the line buried metal body 61, electromotive force Ey induced by detection coil 62 is set to one half of electromotive force E0 expressed with the above-mentioned formula (2) is calculated.
As a result, horizontal displacement distance x from the right above position (x=0) of the line buried metal body 61 becomes equal to burial depth y.
Therefore, it can calculate for burial depth y of line buried metal body 61 by measuring horizontal displacement distance x to which detection coil 62 was moved.
As other methods of measuring burial depth y of line buried metal body 61,
After moving detection coil 62, detecting a depth measurement signal separately and removing the peak value and the minimum value within predetermined time, the detection value of others which remain is averaged and it is considered as the depth measured value which calculates this averaged value.